Encyclopedia of Espionage, Intelligence, and Security

RADAR

█ LARRY GILMAN

RADAR—an acronym for RAdio Detection And Ranging— is the use
of electromagnetic waves at sub-optical frequencies (i.e., less than about
10
12
Hz) to sense objects at a distance. Hundreds of different RADAR systems
have been designed for various purposes, military and other. RADAR systems
are essential to the navigation and tracking of craft at sea and in the
air, weather prediction, and scientific research of many kinds.

Principles.
In basic RADAR, radio waves are transmitted from an antenna. These
outgoing waves eventually bounce off some distant object and return an
echo to the sender, where they are received, amplified, and processed
electronically to yield an image showing the object's location. The
waves sent out may be either short oscillatory bursts (pulses) or
continuous sinusoidal waves. If a RADAR transmits pulses it is termed a
pulse RADAR, whereas if it transmits a continuous sinusoidal wave it is
termed a continuous-wave RADAR.

On closer examination, the RADAR process is seen to be more complex. For
example, reflection of an echo by the object one wishes to sense is
anything but straightforward. Upon leaving a transmitting antenna, a radio
wave propagates in a widening beam at the speed of light (> 186,000
miles per hour [3 × 10
8
m/sec]); if it encounters an obstacle (i.e., a medium whose
characteristic impedance differs from that of air and vacuum [> 377
Ω), it splits into two parts. One part passes into the obstacle and
is (generally) absorbed, and the other is reflected. Where the reflected
wave goes depends on the shape of the obstacle. Roundish or irregular
obstacles tend to scatter energy through a wide angle, while flat or
facet-like surfaces tend to send it off in a single direction, just as a
flat mirror reflects light. If any part of the outgoing wave is reflected
at 180° (which is not guaranteed) it will return to the
transmitter. This returned or backscattered signal is usually detected by
the same antenna that sent the outgoing pulse; this antenna alternates
rapidly between transmitting pulses and listening for echoes, thus
building a realtime picture of the reflecting targets in range of its
beam. The energy the echoes receive is a small fraction of that in the
pulses transmitted, so the strength of the transmitted pulse and the
sensitivity of the receiver determines a RADAR's range. By
systematically sweeping the direction in which its antenna is pointed, a
RADAR system can scan a much larger volume of space than its beam can
interrogate at any one moment; this is why many RADAR antennas, on ships
or atop air-traffic control towers, are seen to rotate while in operation.

Radio waves are not the only form of energy that can be used to derive
echoes from distant targets. Sound waves may also be used. Indeed, because
radio waves are rapidly absorbed in water, sonar (SOund Navigation and
Ranging) is essential to underwater operations of all sorts, including
sea-floor mapping and anti-submarine warfare.

Applications.
Since World War II RADAR has been deployed in many forms and has found a
wide application in scientific, commercial, and military operations. RADAR
signals have been bounced off targets ranging in size from dust specks to
other planets. RADAR is essential to rocketry and early-warning detection
of missiles, air traffic control, navigation at sea, automatic control of
weapons such as antiaircraft guns, aircraft detection and tracking,
mapping of the ground from the air, weather prediction, intruder
detection, and numerous other tasks. Few craft, military or civilian, put
to sea or take to the air without carrying some form of RADAR.

In recent decades, development of the basic RADAR principle—send
pulse, listen for echo—has proceeded along a number of interesting
paths. By exploiting the Doppler effect, which causes frequency shifts in
echoes reflected from moving objects, modern RADARs can tell not only
where an object is but what direction it is moving
in and how quickly. The Doppler effect also allows for the precision
mapping of landscapes from moving aircraft through the synthetic-aperture
technique. Synthetic-aperture systems exploit the fact that stationary
objects being swept by a RADAR beam projected from a moving source have,
depending on their location, slightly different absolute velocities with
respect to that source. By detecting these velocity differences using the
Doppler effect, synthetic aperture type RADAR greatly permits the
generation of high-resolution ground maps from small, airborne RADARs.

In many modern RADAR systems the need for a mechanically moving antenna
has been obviated by phased arrays. A phased array consists of a large
number of small, computer-controlled antennas termed elements. These
elements, of which there are usually thousands, are crowded together to
form a flat surface. In transmit mode, the elements are all instructed to
emit a RADAR pulse at approximately the same time; the thousands of
outbound waves produced by the elements merge into a single powerful wave
as they spread outward. By timing, or
phasing,
the elements in the array so that, for example, elements along the
left-hand edge of the array fire first while those farther to the right
fire progressively later, the composite wave formed by the merging of the
elements' lesser outputs can be steered in any desired direction
within a wide cone (in this example, to the right). Beam steering can be
accomplished by such a system millions of times more rapidly than would be
possible with mechanical methods. Phased-array systems are used for a
number of applications; including the 71.5-foot (21.8-m) tall AN/FPS-115
PAVE PAWS Early Warning RADAR Array Antennas, which provide early warning
of ballistic-missile attack; shipboard systems such as the AN/SPY-1D,
which is about 15 feet (3 m) across and is mounted flush with the upper
hull of some warships; the Hughes AN/TPQ-37 Firefinder, a trailer-mounted
system designed for tracking incoming artillery and missiles and
calculating their point of origin; and many other real-world systems.

RADAR is a powerful weapon of war, but has its weaknesses. For example,
numerous missiles have been developed to home in on the radio pulses
emitted by RADARs, making it very dangerous to turn on a RADAR in a modern
battlefield situation. Further, jamming and spoofing ("electronic
warfare") have evolved rapidly alongside RADAR itself. For example,
an aircraft that finds itself interrogated by a RADAR pulse can emit
blasts of noise or false echoes, or request that a drone or other unit
emit them, in order to confuse enemy RADAR. Finally, aircraft have been
built that are "low observable," that is, which scatter very
little energy back toward any RADAR that illuminates them. Low-observable
or "stealth" aircraft are built of radio-absorbent materials
and shaped to present little or no surface area perpendicular to RADAR
pulses approaching from most angles (except directly above and directly
below, the two least likely places for an enemy RADAR to be at any given
moment). What RADAR they do reflect is deflected at low angles rather than
returned to the RADAR transmitter. The U.S. B-2 bomber and F-117A and F-22
fighters are working examples of low-observable aircraft.